Fuel cells need not to be exchanged or charged, unlike general-purpose cells, and are power-generation cells that convert chemical energy into electrical energy using a fuel such as hydrogen or methanol.
Also, the fuels cells are high-efficiency power generation systems having an energy conversion efficiency of about 60% and their fuel consumption is lower than conventional internal combustion engines due to their high efficiency. In addition, they are nonpolluting energy sources that do not generate environmental pollutants such as SOx, NOx, VOC, etc. In addition, these fuel cells have advantages in that they can use various fuels and require a small area and a short construction period.
Due to these advantages, the fuel cells are used in various applications, including mobile power sources such as portable devices, transportable power sources such as automobiles, distributed power generation systems which can be used for home and power business applications, etc. Particularly, it is expected that, if fuel cell automobiles that are next-generation transportation systems are put to practical use, the potential market size of the fuel cells will be enormous.
Fuel cells are largely classified, according to operating temperature and electrolyte, into alkali fuel cells (AFCs), phosphoric acid fuel cells (PAFCs), molten-carbonate fuel cells (MCFCs), solid oxide fuel cells (SOFCs), polymer electrolyte membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs).
Among them, the polymer electrolyte membrane fuel cell and the direct methanol fuel cell, which have excellent mobility, are receiving a great deal of attention as next-generation power sources.
An electrolyte membrane that is the key element of the polymer electrolyte membrane fuel cell and the direct methanol fuel cell should function not only to transfer hydrogen ions, but also to prevent fuel from moving from a positive electrode to a negative electrode. Thus, the electrolyte membrane acts as a cation exchange membrane and should have hydrogen ion conductivity and, at the same time, chemical, thermal, mechanical and electrochemical stabilities.
Typical examples of ion conductive polymer electrolyte membranes include Nafion, a perfluorinated hydrogen-ion exchange membrane developed by DuPont (USA) in the early 1960s. In addition, commercial perfluorinated polymer electrolyte membranes similar to Nafion include Aciplex-S membrane (Asahi Chemicals), Dow membrane (Dow Chemicals), Flemion membrane (Asahi Glass), etc.
Although the commercial perfluorinated polymer electrolyte membranes have chemical resistance, oxidation resistance and excellent ion conductivity, they are expensive and the toxicity of intermediate products occurring during the manufacture of the membrane causes environmental problems.
In order to overcome the drawbacks of such perfluorinated polymer electrolyte membranes, polymer electrolyte membranes comprising aromatic polymers having a carboxyl group, a sulfonic acid group or the like introduced therein have been studied. Examples thereof include sulfonated polyarylether sulfones [Journal of Membrane Science, 1993, 83, 211], sulfonated polyether ketones [Japanese Patent Laid-Open Publication No. Hei 6-93114, and U.S. Pat. No. 5,438,082], sulfonated polyimides [U.S. Pat. No. 6,245,881], etc.
However, such polymer electric membranes have problems in that a dehydration reaction is likely to occur by an acid or heat in the process of introducing a sulfonic acid group onto the aromatic ring and in that the hydrogen ion conductivity of the membrane is greatly influenced by water molecules.
The above-mentioned perfluorinated and hydrocarbon-based polymer electrolyte membranes show a rapid decrease in ion conductivity due to a decrease in water content at a temperature higher than 100° C., become soft above 100° C. and have high methanol permeability. Due to such problems, the polymer electrolyte membranes are very difficult to commercialize.
In an attempt to overcome such problems, studies on reinforced composite electrolyte membranes are being actively conducted in many companies. For example, Gore Select® membranes are currently commercially available from W. L. Gore & Associates, Inc (USA).
The performance of the reinforced composite electrolyte membranes is determined according to the chemical structure, pore size, porosity and mechanical properties of a porous substrate employed to impart mechanical properties and dimensional stability to the membrane.
Thus, in order to satisfy the requirement of the high ion conductivity of the reinforced composite electrolyte membrane, the porous substrate is required to have a high porosity of 50% or higher.
However, if the porous substrate has high porosity, the mechanical properties of the membrane will be reduced. Accordingly, there is an urgent need to develop a porous substrate satisfying all the requirements of high porosity and excellent mechanical properties.
Meanwhile, polytetrafluoroethylene or polyethylene porous substrates which are in common use are hydrophobic in nature, and thus have low miscibility with hydrogen ion conducting electrolytes, so that the electrolyte is difficult to impregnate into the porous substrate and the interfacial adhesion therebetween is reduced.
In an attempt to solve this problem, studies on making polytetrafluoroethylene hydrophilic by surface modification have been reported [Electrochimica Acta, 2007, 52, 5304, J. Materials Chemistry, 2007, 4, 386, J. Membrane Science, 2007, 306, 298].
However, although this technology provides some improvement in the hydrophilicity of the membrane, it does not provide a fundamental improvement in the hydrophilicity due to the property of the material of the membrane. For this reason, post-treatment processes such as plasma treatment are applied. This post-treatment process causes damage to the porous substrate and makes the manufacture process complex. Accordingly, there is a need to develop a new conceptual porous substrate which is completely different from conventional porous substrates.